Super-adsorptive and photo-regenerable carbon nanotube based membrane for highly efficient water purification
Graphical abstract
Introduction
Water pollution by various organic and inorganic contaminants from industrial, agricultural and pharmaceutical wastewater has worsened the global challenge of water scarcity [[1], [2], [3]]. Among these contaminants, dyes [4], pharmaceuticals [5], and pesticides [6] have been often detected in drinking water. After disinfecting, some organic contaminants transfer into highly toxic disinfection by-products via chlorine substitution or addition reaction, inducing acute or chronic diseases for humans or animals [7,8]. Therefore, developing advanced treatment technologies for efficient organic contaminants removal is of special interest and great importance. Various methods, such as coagulation/flocculation, adsorption, advanced oxidization processes (AOPs) and membrane filtration, have been implemented to reduce organic contaminants [[9], [10], [11], [12]].
Among these methods, membrane separation has been recognized as the next generation of water purification and reclamation technology due to its high separation efficiency, continuous and simple operation, less chemical requirements, modularity and low footprint [9,13]. However, highly selective pressure driven membranes capable of removing organic micropollutants are often dense, and require high hydraulic pressure and thus high energy consumption [14,15]. Membrane distillation and forward osmosis could be promising for removing organic micropollutants due to their minimal pressure requirements, but the former is limited to non-volatile organic micropollutants [16] and the latter requires regeneration of the draw solution [17]. Meanwhile, traditional membranes suffer from fouling during long-term operation, leading to temporary or permanent declines in flux and rise in energy consumption [18]. Therefore, new membrane materials preparation and design have been widely studied to overcome the trade-off between water purification efficiency and energy consumption.
Carbon nanotubes (CNTs) are promising materials for water purification. CNTs have been used as adsorbents [19] and catalyst-supports [20] in AOPs for various wastewater treatment applications. CNTs have been combined with different materials to form nanocomposites, such as CNTs-bentonite [21], CNTs-graphene [20], and CNTs-magnetic graphene [22] for pollutants adsorption. Reactive metals and oxides of Fe, Ti, Cu, or Mn have also been employed to load on CNTs, thereby inducing diverse superoxides to produce highly reactive oxygen species (ROSs), such as hydroxyl radical (•OH) and/or superoxide radical (O2•) [20,23]. The reactive radicals can degrade a wide range of toxic organic pollutants into harmless mineralized salts, carbon dioxide, and water, leading to the regeneration of CNTs. However, the adsorption capacity of CNTs based adsorbents are limited, and CNTs supported catalysis requires chemicals for ROSs generation and catalysts regeneration. On the other hand, CNTs as powders tend to cause serious recontamination and face recycling issues [24,25].
CNTs based membranes have been regarded as attractive candidates for water purification [26,27]. CNT membranes with interweaved nanoporous structures have been fabricated by a combination of self-assembly and simple filtration methods [28,29]. Utrathin free-standing CNTs network membranes were also developed via the facile vacuum filtration method for separation of emulsified oil/water mixture. However, most of these CNT membranes were used for water purification based on their adsorptive or rejection properties, in which membrane fouling and performance regeneration are the key challenges [18,30,31]. In addition, these CNT membranes cannot be used to remove trace organic contaminants with relatively small molecular weights from drinking water due to their large pore sizes.
Engineering adsorptive and catalytic CNT based membranes would solve the disadvantages of conventional CNT based adsorbents and membranes mentioned above, and lead to highly efficient removal of organic micropollutants due to the synergetic effect of physical adsorption and chemical catalysis [32,33]. In this work, we developed an interweaved super-adsorptive and photo-regenerable CNT based membrane for highly efficient water purification. First, a highly adsorptive CNT membrane was prepared by dispersing single wall CNT powders in a water solution followed by vacuum filtration. The CNT membrane was then used as a support to anchor photocatalytic FeOOH catalysts on the outer walls of CNTs, endowing the CNT based membrane with excellent self-cleaning properties via photo-induced Fenton-like oxidation. Finally, to improve the membrane stability under practical conditions, we utilized the electroless welding method [34] to produce silver nano-knots between the adjacent interweaving CNTs by the silver-ammonia reaction. The adsorption, dynamic filtration, and photo-regeneration (i.e. self-cleaning) performances of the CNT based membranes in water purification were evaluated using different model pollutants (e.g. Rhodamine B, methylene blue, and eosin Y). This study provides a new strategy to engineer multifunctional high performance membranes with adsorptive and self-cleaning properties for water purification and wastewater reclamation.
Section snippets
Materials and chemicals
Electrospun polypropylene (PP) flat-sheet membranes (mean pore size 0.22 μm, thickness ~8 μm, SEM images were shown in Fig. S1) from Haining Kewei Filtration Equipment Co. Ltd. (Zhejiang, China) were used as the support for CNT filtration. Single-walled CNTs (diameters 10–30 nm; lengthens 10–30 μm, purity > 99%) were purchased from Xianfeng Nano Materials Tech Co., Ltd (Jiangsu, China). Silver nitrate, ammonia solution, dimethyl formamide (DMF), ferrous sulfate tetrahydrate, hydrogen peroxide (H
Membrane fabrication and characterization
Combining CNTs with membranes has several advantages. First, CNTs can act as both adsorbents and catalyst-supports, imparting the membranes with excellent adsorptive and catalytic properties after loading with catalysts [19,20] for various wastewater treatment applications. Second, the CNT based catalytic membranes can not only degrade a wide range of toxic organic pollutants, but also reduce membrane fouling by the degradation-induced self-cleaning properties [37]. Last, selecting membranes as
Conclusions
In this work, we developed a highly adsorptive self-cleaning carbon nanotube based membrane (Ag-CNT-FeOOH membrane) via the facile vacuum filtration method followed by silver-amino reaction. The Ag-CNT-FeOOH membrane showed improved robustness due to the welding of nanoparticles on the membrane surface by silver-amino reaction. In dynamic filtration, our membrane exhibited superior adsorption performance among CNT based adsorbents reported in literature. For example, the adsorption capacities
CRediT authorship contribution statement
Yang Yang: Data curation, Investigation, Methodology, Writing - original draft. Zhu Xiong: Data curation, Investigation, Funding acquisition, Writing - review & editing. Zhu Wang: Investigation, Methodology, Writing - review & editing. Yi Liu: Methodology, Conceptualization, Writing - review & editing. Zijun He: Investigation, Methodology. Akun Cao: Data curation, Methodology. Li Zhou: Methodology. Lijing Zhu: Data curation, Methodology. Shuaifei Zhao: Methodology, Conceptualization,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (51808142, 51538013), the National Key Research and Development Plan (2016YFA0203200), the introduced innovative R & D team project under the “The Pearl River Talent Recruitment Program” of Guangdong Province, Science and Technology Research Project of Guangzhou (201904010217), State Key Laboratory of Pollution Control and Resource Reuse Foundation (PCRRF19010), Scientific Project of Guangzhou University (YG2020020) and
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These authors contributed equally to this work.